EP1421393B1 - Optical current sensors - Google Patents

Abstract

An optical current or magnetic field sensor has a sensor head which has a first phase delay element (13), a second phase delay element and a sensor fiber (15) The two phase delay elements (13) are optically connected to opposite ends of the sensor element (15), and the phase delay angle rho on at least one of the phase delay elements (13) deviates by an angle epsilon, with epsilon<>0°, from 90°. Linearly polarized light waves (3) are injected into the first phase delay element (13), with a polarization axis (y') of these linearly polarized light waves (3) including an angle which deviates from 45° by an angle Deltaalpha, with Deltaalpha<>0°, with a principal axis (y) of the first phase delay element (13). The angle Deltaalpha is selected as a function of at least the angle epsilon. This choice of the angle Deltaalpha makes it possible to compensate for non-linearities between a direct measurement signal and an electrical current or magnetic field to be measured which occur because of the angle epsilon<>0°.

Description

• ein Verfahren zur Herstellung eines Sensors gemäss dem Oberbegriff des Patentanspruchs 1,
• auf einen optischen Strom- oder Magnetfeldsensor gemäss dem Oberbegriff des Patentanspruchs 10 und
• auf ein Verfahren zur Messung eines elektrischen Stroms oder Magnetfeldes gemäss dem Oberbegriff des Patentanspruchs 11.

The present invention relates to the field of optical current or magnetic field sensors. It refers to

• a method for producing a sensor according to the preamble of patent claim 1,
• to an optical current or magnetic field sensor according to the preamble of patent claim 10 and
• to a method for measuring an electric current or magnetic field according to the preamble of patent claim 11.

State of the art

An optical current sensor is known from EP 0 856 737 A1. He has a coil-shaped, magneto-optically active sensor fiber, which encloses a conductor. At least at one end is the sensor fiber via a phase delay element with a further optical Fiber, a so-called supply or return fiber, connected, via which light can be coupled into or out of the sensor fiber leaves. The feed or return fibers are preferably an elliptical core cross-section. In them propagate linearly polarized Light waves. As a phase delay element acts a birefringent Fiber segment, which is arranged between sensor fiber and supply fiber is. This fiber segment has two main optical axes, one fast (short) and slow (long) main axis. These are below 45 ° aligned to the two main axes of the supply and return fibers. Typically, its length is chosen to function as a λ / 4 phase delay element acts according to a phase delay angle of an odd multiple of 90 °. As a result, it transforms the aforementioned linearly polarized light waves of the feed and return fibers in circularly polarized light waves, which propagate in the sensor fiber. For the mentioned angle between the main axes and for the phase delay angle the phase delay element are in the mentioned EP 0 856 737 A1 tolerance angle indicated.

The sensor fiber is used either as a Sagnac interferometer or, if one their ends is mirrored, operated as a reflection interferometer. In both Cases spread in the sensor fiber two circularly polarized light waves out. The waves are opposite in the case of the Sagnac interferometer, in the case of the reflection interferometer, they run in the same direction. In the Sagnac interferometer both waves have the same polarization sense, where they are either left or right circularly polarized. In the reflection interferometer they have an opposite polarization sense on.

If the current conductor is traversed by an electric current I, then the current I generates a magnetic field which leads to a differential phase shift between these two countercurrent light waves. This effect is called magneto-optic effect or Farraday effect. For circularly polarized light waves in the sensor fiber, the resulting phase shift is proportional to the current and is in the Sagnac configuration Δ S = 2 φ F and in the reflection configuration Δ R = 4 φ F . wherein the so-called Faraday phase shift φ _F as φ F = VNI, where V is the Verdet constant of the sensor fiber, N is the number of turns of the coil and I is the current.

In the cited EP 0 856 737 A1 a sensor fiber is described which free of mechanical stresses, so that the received measuring signal not from a temperature-dependent, stress-induced linear birefringence is disturbed. However, the Verdet constant V of the sensor fiber has also a temperature dependence which, even with an ideal, stress-free fiber coil is noticeable.

In EP 1 115 000, a fiber optic current sensor is described, the influence suppresses the temperature dependence of the Verdet constant V, in that a phase delay element is used whose temperature dependence the temperature dependence of the Verdet constant V compensated. This is achieved by the phase delay element a Phase delay angle, which by an angle ε ≠ 0 ° of the phase delay angle of an ideal phase delay element differs. For a λ / 4 phase delay element, the phase delay angle is instead of 90 ° then 90 ° + ε, corresponding phase delay angles typically 95 ° to 105 °. Depending on the sign of the temperature dependence The Verdet constants V are also negative angles ε possible.

Such phase delay elements are preferably as birefringent Fiber segments formed with elliptical core cross-section, wherein the phase delay angle then by appropriate choice of the length of the fiber segment can be easily adjusted. Become one Phase delay element with ε ≠ 0 ° linearly polarized over the supply fiber Light waves supplied, the main axes of the supply fiber include an angle of 45 ° with those of the phase delay element, so slightly elliptically polarized in the sensor fiber Light waves off.

A problem with this type of compensation of the temperature dependence of the Verdet constant V is that it results in a deviation from a linear relationship between the current to be measured and a measurement signal, the measurement signal being generally proportional to Δ _S or Δ _R is. This means that the valid linear relationships for circularly polarized light waves Δ _S = 2 VNI and Δ _R = 4 VNI are no longer valid for elliptical light waves. Such nonlinearities of the ratio between the current to be measured and a measurement signal typically have a magnitude of 0.1% to 1% and cause measurement inaccuracies or complicate the evaluation of the measurement, if greater measurement accuracy is required. With the help of a complex signal processing such nonlinearities can be compensated.

Presentation of the invention

It is an object of the invention to provide an improved current or magnetic field sensor of the type mentioned, a corresponding manufacturing process and a corresponding measurement method to accomplish. The sensor should solve the above mentioned disadvantages. Especially the sensor should have a greater accuracy of measurement and / or facilitate the evaluation of the measurement and the use of a unnecessary signal processing.

This object triggers a method for producing a optical current or magnetic field sensor having the features of the claim 1, an optical current or magnetic field sensor according to claim 10 and a method for measuring an electric current or a magnetic field with the features of claim 11.

The optical current or magnetic field sensor has a sensor head with a sensor element and two phase delay elements as well two light pipe elements. The components of the sensor head are in the Sequence first light pipe element, first phase delay element, Sensor element, second phase delay element, second light pipe element arranged along a light path and optically interconnected. The phase delay angles ρ, ρ 'of the phase delay elements soft by angle ε, ε '≠ 0 ° with -90 ° <ε, ε' <90 ° of an odd multiple of 90 °. A major axis of the first light pipe element closes with a Main axis of the adjacent first phase delay element a Angle of 45 ° ± Δα with Δα ≠ 0 ° and 0 ° <Δα <45 °, which depends on is selected from the angles ε, ε '. Nonlinearities between an immediate measurement signal and an electrical current to be measured Current or magnetic field, which result from ε, ε '≠ 0 °, can by a corresponding ε, ε'-dependent choice of the angle Δα at least approximately be compensated. This creates an at least approximate linear relationship between the immediate measurement signal and the electric current or magnetic field to be measured. This has the advantage of a to allow easy evaluation of the measurement, and greater measurement accuracy reach without consuming a complex signal processing.

In the inventive method for producing a optical current or magnetic field sensor are called Components of the sensor head arranged and dimensioned in the manner mentioned. In particular, said angle Δα is dependent on the angles ε, ε 'chosen.

A Sagnac-configured sensor is inexpensive can be produced, since a commercially available detection unit without consuming Adjustments can be used.

In an advantageous embodiment of the invention, the angle Δα is dependent of ε, ε 'selected such that the said nonlinearities um at least half an order of magnitude, ie a factor of 3, compared to Case Δα = 0 ° are reduced.

In a preferred embodiment of the invention, both have phase delay elements a non-zero deviation ε, ε 'of their Phase delay angle ρ, ρ 'of odd multiples of 90 °, and both phase delay elements together have a temperature dependence on that the temperature dependence of the Verdet constants V of the sensor element at least approximately compensated. The means, the two phase delay elements together provide one such contribution to the temperature dependence of the instantaneous measurement signal, that the temperature dependence of the Verdet constant V of the sensor element is at least approximately compensated. The ones from the combination the two phase delay elements resulting temperature dependence is thus chosen in the described way. In this way, the Sensor not only an at least approximately linear relationship between the immediate measurement signal and the electrical to be measured Current or magnetic field, but also improved temperature stability.

In a further preferred embodiment of the subject invention In addition, the phase delay angles ρ, ρ 'are equal (ρ = ρ' and ε = ε '). And the relative alignment of the phase delay elements too is the optically associated with each optical fiber elements selected the same, that is, for both phase delay elements at least one major axis of the phase delay element with at least a major axis of the light pipe element the same angle Include Δα = Δα '. Due to this symmetrical design of the sensor head is an improved manufacturability and a great insensitivity achieved against interference.

Furthermore, it is advantageous if the coupling of the linearly polarized Light waves in the phase delay elements by means of polarization-preserving Fibers as light pipe elements takes place. This can cause the generation of the linearly polarized light waves in a large spatial distance from the Phase delay elements and the sensor element to be arranged while still the coupling of linearly polarized light waves always takes place at the same angle.

In another preferred embodiment of the subject invention is at least one of the two phase delay elements, preferably both phase delay elements, as fiber piece with elliptical core formed, which has a phase delay angle of 90 ° + ε (or 90 ° + ε '). Such phase delay elements are simple and inexpensive to produce.

It is advantageous to choose the sensor element so that it has a current conductor can include coil-shaped, because so can the measurement accuracy and sensitivity of the sensor can be increased.

It is also advantageous, as a sensor element, a magneto-optically active To use glass fiber, which is almost free of mechanical stresses. As a result, a temperature dependence of the sensor is reduced.

Further preferred embodiments are based on the patent claims out.

Brief description of the drawings

Fig. 1

Schematische Darstellung eines Teils eines erfindungsgemässen Sensors;

Fig. 2

Schematische Darstellung eines erfindungsgemässen Strom- oder Magnetfeldsensors in Sagnac-Konfiguration;

Fig. 3

Schematische Darstellung der Ausbreitungsrichtung von sich im Betrieb eines erfindungsgemässen Strom- oder Magnetfeldsensors im Sensorkopf ausbreitenden Lichtwellen;

Fig. 4

Graphische Darstellung des berechneten Zusammenhangs zwischen der differentiellen Phasenverschiebung Δ_S (normiert auf 2 ϕ_F) und dem Zweifachen der Faraday-Phasenverschiebung ϕ_F, 2 ϕ_F = 2 V N I, bei zueinander parallel ausgerichteten schnellen Achsen der Phasenverzögerungselemente (χ=0°) und Δα = Δα' = 0°, für verschiedene Winkel ε;

Fig. 5

Graphische Darstellung des berechneten Zusammenhangs zwischen der differentiellen Phasenverschiebung Δ_S (normiert auf 2 ϕ_F) und dem Zweifachen der Faraday-Phasenverschiebung ϕ_F, 2 ϕ_F = 2 V N I, bei zueinander parallel ausgerichteten schnellen Achsen der Phasenverzögerungselemente (χ=0°) und ε = 0°, für verschiedene Winkel Δα = Δα';

Fig. 6

Graphische Darstellung des berechneten Zusammenhangs zwischen der differentiellen Phasenverschiebung Δ_S (normiert auf 2 ϕ_F) und dem Zweifachen der Faraday-Phasenverschiebung ϕ_F, 2 ϕ_F = 2 V N I, bei zueinander parallel ausgerichteten schnellen Achsen der Phasenverzögerungselemente (χ=0°) und ε = 13°, für verschiedene Winkel Δα = Δα';

In the following the subject invention will be explained in more detail with reference to preferred embodiments and the accompanying drawings. Show it:

Fig. 1

Schematic representation of a part of a sensor according to the invention;

Fig. 2

Schematic representation of a current or magnetic field sensor according to the invention in Sagnac configuration;

Fig. 3

Schematic representation of the propagation direction of propagating in the operation of a current or magnetic field sensor according to the invention in the sensor head propagating light waves;

Fig. 4

Graphical representation of the calculated relationship between the differential phase shift Δ _S (normalized to 2 φ _F ) and twice the Faraday phase shift φ _F , 2 φ _F = 2 VNI, with parallel fast axes of the phase delay elements (χ = 0 °) and Δα = Δα '= 0 °, for different angles ε;

Fig. 5

Graphical representation of the calculated relationship between the differential phase shift Δ _S (normalized to 2 φ _F ) and twice the Faraday phase shift φ _F , 2 φ _F = 2 VNI, with parallel fast axes of the phase delay elements (χ = 0 °) and ε = 0 °, for different angles Δα = Δα ';

Fig. 6

Graphical representation of the calculated relationship between the differential phase shift Δ _S (normalized to 2 φ _F ) and twice the Faraday phase shift φ _F , 2 φ _F = 2 VNI, with parallel fast axes of the phase delay elements (χ = 0 °) and ε = 13 °, for different angles Δα = Δα ';

The reference numerals used in the drawings and their meaning are listed in the list of references summarized. in principle In the figures, like parts are given the same reference numerals. The described embodiments are exemplary of the subject invention and have no restrictive effect.

Ways to carry out the invention

FIG. 1 shows schematically a part of a sensor head 1 for an optical current or magnetic field sensor. A first light pipe element 11, which is used as a polarization-maintaining optical fiber with elliptical Core cross-section is formed, with a first end 131 of a first phase delay element 13 optically connected, i. light waves can from the light pipe element 11 in the first end 131 of first phase delay element 13 are coupled and vice versa. As a first phase delay element 13 is a piece of fiber with elliptical Core cross section used, whose length is chosen such that its phase delay angle ρ = 90 ° + ε with an angle ε ≠ 0 °. From the aforementioned EP 1 115 000 is known to be a skillful Choice of the angle ε to improve the temperature stability of an optical Current sensor can lead. A second end 132 of the first phase delay element 13 is optically connected to a sensor element 15, which is preferably a magneto-optically active fiber with a round core cross-section.

In the three graphs in the upper part of Figure 1 are light waves 3,4 _x , 4 _y , 6, which propagate in the three fiber segments mentioned 11,13,15, and preferred core cross-sections and major axes x, x ', y, y' shown schematically. The light waves are represented by thick arrows that symbolize the E-field vectors of the light waves. First linearly polarized light waves 3, which in this case have a polarization axis y 'which coincides with the slow, long main axis y' of the first light-conducting element 11, propagate in the first light-conducting element 11. The main axis y 'of the first light-conducting element 11 encloses an angle of 45 ° + Δα with a main axis y of the first phase-delay element 13, where Δα ≠ 0 ° is an angle that is selected as a function of the angle ε. When the first linearly polarized light waves 3 enter the first phase delay element 13, they become second linearly polarized light waves 4, comprising second linearly polarized light waves 4 _x and second linearly polarized light waves 4 _y , their polarization axes along the two main axes x, y of the first phase delay element 13 lie.

The two graphs in the lower part of Figure 1 schematically illustrate the second linearly polarized light waves 4 in the first phase delay element 13. At the first end 131 of the first phase delay element 13, the second linearly polarized light waves polarized along the slow main axis y of the first phase delay element 13 are 4 _y with the second linearly polarized light waves 4 _x in phase polarized along the fast main axis x of the first phase delay element 13. After passing through the first phase delay element 13 along the propagation direction z, the phase difference ρ = 90 ° + ε has accumulated at the second end 132 of the first phase delay element 13 between the light waves 4 _x and the light waves 4 _y .

By the coupling of the second linearly polarized light waves 4 in the Sensor element 15 at the second end 132 of the phase delay element 13 arise there elliptically polarized light waves 6, which then propagate in the sensor element 15.

FIG. 2 shows a current or magnetic field sensor according to the invention has a Sagnac configuration. On the basic structure and on the operation of the sensor will not be discussed in detail here. The cited prior art can provide appropriate information be removed. In addition to the sensor head 1 has the current or Magnetic field sensor still has a transmitter-evaluation unit 2. This includes In the example shown, a light source 20, a fiber coupler 21, a Fiber polarizer 22, a second fiber coupler 24 and a phase modulator 25, and a detector 26, a signal processor 27 and a measured value output 28. The transmitter-evaluation unit 2 is used to generate and Detection of light and the evaluation and output of measured data.

In FIG. 3, the directions of propagation are those of the operation shown in FIG Current or magnetic field sensor in the sensor head 1 capable of propagation Light waves shown. Give the open arrows by the reference sign the propagation direction. In FIG. 2, for reasons of clarity only a few of the light waves and propagation directions are shown. For the following explanations, reference is made to FIGS. 2 and 3.

Via the first light-conducting element 11 and a second light-conducting element 12 or entprechende extensions or connections is the transmitter-evaluation unit 2 connected to the sensor head 1. The latter shows beside the first phase delay element 13 nor a second phase delay element 14, which is analogous to the first phase delay element 13 at a first end 141 with the second light pipe element 12 is optically connected, and at a second end 142 with a second end of the sensor element 15 is optically connected. The sensor element 15 is formed as a magneto-optically active fiber, the coil-shaped a current conductor S encloses. The light pipe elements 11 and 12 are polarization-maintaining fibers with an elliptical core cross-section educated.

In the transmitter-evaluation unit 2 linearly polarized light is generated, from Which then in the two light pipe elements 11 and 12, the first linear polarized light waves 3 and 3 'are. The graphics in the middle of Figure 2 symbolize these light waves 3,3 'as thick arrows. The open Arrows indicate the propagation direction of the light waves 3,3 '.

The first linearly polarized light waves 3 from the first light pipe element 11, as described in connection with Figure 1, means of the (first) phase delay element 13 in elliptically polarized light waves 6, which then propagate in the sensor element 15. there closes a main axis of the first light pipe element 11 with a Main axis of the first phase delay element 13 the mentioned Angle 45 ° + Δα, and the phase delay angle ρ of the first phase delay element 13 is the stated ρ = 90 ° + ε.

If the current conductor S is traversed by an electric current I, the elliptically polarized light waves 6 experience a magneto-optically induced phase shift due to the Faraday effect. After passing through the sensor fiber, the elliptically polarized light waves 6 couple into the second end 142 of the second phase retarding element 14 where they are converted to third linearly polarized lightwaves 5 comprising linearly polarized lightwaves 5 _x and 5 _y . These third linearly polarized light waves 5 excite fourth linearly polarized light waves 5 a in the second light conduction element 12, which are then supplied via the second light conduction element 12 to the transmit-evaluation unit 2. There, the light waves are detected. Third light waves 5a, whose polarization axis is oriented perpendicular to the polarization axis of the first linearly polarized light waves 3, can be blocked in the fiber polarizer 22 so that they are not detected.

The same applies to the first linearly polarized light waves 3 'in the second light pipe element 12. The resulting light waves are provided with primed reference numerals. The first linearly polarized Light waves 3 'are detected by means of the second phase delay element 14 converted into elliptically polarized light waves 6 ', which then in the sensor element 15 propagate, with a propagation direction, that of the elliptically polarized light waves 6 is opposite. In this case, a main axis of the second light-conducting element 12 includes a major axis of the second phase retarding element 14 an angle 45 ° + Δα ', with an angle Δα', for which 0 ° ≤ Δα '<45 °. The second phase delay element 14 has a phase delay angle ρ 'is different on the by an angle ε' of 90 °, that is ρ '= 90 ° + ε'. It is ε '≠ 0 °.

A magnetic field, which arises due to an electric current I flowing in the current conductor S, causes a magneto-optically induced phase shift of the elliptically polarized light waves 6 'via the Faraday effect. After passing through the sensor element 15, the elliptically polarized light waves 6 'couple into the second end 132 of the first phase delay element 13. Third linearly polarized light waves 5 'are generated, which comprise third linearly polarized light waves 5 _x ' and 5 _y ', and there is a phase shift of 90 ° + ε between these third linearly polarized light waves 5 _x ' and 5 _y '. Then arise at the first end 131 of the first phase delay element 13 fourth linearly polarized light waves 5a ', which are then fed via the first optical fiber element 11 of the transmitter-evaluation unit 2. In this, the light waves are detected, and the magneto-optically induced phase shift is determined. Third light waves 5a 'whose polarization axis is oriented perpendicular to the polarization axis of the first linearly polarized light waves 3' can be blocked in the fiber polarizer 22 so that they are not detected. The magneto-optically induced phase shift from the elliptically polarized light waves 6 or from the elliptically polarized light waves 6 'propagating in opposite directions in the sensor element 15 can serve as an immediate measurement signal for determining the electrical current I. The term "immediate" measurement signal is intended to mean that no signal conditioning has taken place in order to generate a signal which is at least approximately proportional to the electrical current I from the measurement signal.

For easy determination of the magneto-optically induced phase shift, the signal of one elliptically polarized lightwave 6 is preferably used in a sensor in Sagnac configuration as a reference signal for the other counter-rotating elliptically polarized lightwaves 6 '. As a direct measurement signal then a differential phase shift ΔΦ _{S of} the two elliptically polarized light waves 6 and 6 'is taken. This direct measurement signal is exactly twice as large as the magneto-optically induced phase shift, which experiences each of the elliptically polarized light waves 6 and 6 'individually. In the case known from EP 0 856 737 A1 ε = 0 °, ε '= 0 °, Δα = 0 °, Δα' = 0 °, this direct measurement signal then amounts to twice the Faraday phase shift φ _F , 2φ _F = 2 VNI, and is thus proportional to the electric current I.

If, as in said EP 1 115 000, ε ≠ 0 ° and ε '≠ 0 °, but Δα = 0 ° and Δα' = 0 °, then the relationship between the immediate measurement signal and the electric current I no longer linear. If one describes the propagation of the elliptically polarized light waves 6,6 'with the aid of Jones matrices and one derives their differential phase shift Δ _S from the complex amplitudes of the two in the detector 26 in the transmitter-evaluation unit 2 interfering light waves, so one obtains for the immediate measurement signal Δ _S Δ S = arctane (cosε + cosε ') sin (2φ F ) (1 + cosε cosε ') cos (2φ F ) - sinε sinε 'cos (2χ)

In this case, χ is the angle between the fast axes of the two phase delay elements 13 and 14. For the special case χ = 0 ° and ε = ε 'this results for small Faraday phase shifts φ _F approximately Δ _S = 2φ _F / cosε, which is for small angle ε as Δ _S = 2φ _F (1 + ε ^2/2) can be approximated. For the special case χ = 90 ° and ε = ε ', this gives for small φ _F approximately Δ _S = 2φ _F cosε what ε for small angles than Δ _S = 2φ _F (1-ε ^2/2) can be approximated ,

For ε = ε '= 13 ° and χ = 0 °, the relative deviation from the linearity of the ratio between the instantaneous measurement signal Δ _S and the electric current I to be measured is approximately -0.21% at 2φ _F = 40 ° and 0.92% at 2φ _F = 90 °. So far, Δα = 0 ° and Δα '= 0 ° were assumed, ie the main axes of the light-conducting elements 11 and 12 each include an angle of exactly 45 ° with the main axes of the phase-delay elements 13 and 14.

If, according to the inventive case, ε, ε '≠ 0 °, and in addition Δα ≠ 0 ° and / or Δα' ≠ 0 ° is selected, then the relationship between the immediate measurement signal .DELTA. _S and that to the electrical Current I proportional Faraday phase shift φ _F Δ S = arctane ∟ (cosε cos (2Δα) + cosε 'cos (2Δα') ┘sin (2φ F ) A 1 cos (2φ F ) + A 2 cos (2χ) - A 3 sin (2χ) With A 1 = 1 + cos ε cos ε 'cos (2Δα) cos (2Δα') A 2 = sin (2Δα) sin (2Δα ') - sin ε sin ε' cos (2Δα) cos (2Δα ') A 3 = sin ε cos (2Δα) sin (2Δα ') + sin ε' cos (2Δα ') sin (2Δα)

Can be obtained by the choice of the angles Δα and Δα 'Thus, as in Fig. 6 can be seen, the non-linear relationship between the direct measurement signal Δ _S and the electric current 1 are influenced so that the non-linearity is reduced. A clever choice of the angles Δα and Δα 'as a function of the angles ε and ε' makes it possible to compensate at least approximately for these nonlinearities resulting from ε ≠ 0 ° and ε '≠ 0 °. Then there is an at least approximately linear relationship between the direct measurement signal Δ _S and the electric current I. In general, the non-linearity can be reduced by at least half a magnitude, ie a factor of 3, compared to the case Δα = Δα '= 0 °. A mathematically complete compensation of the nonlinearity is not possible because of the difference in the functional relationships for the angles ε, ε 'and for the angles Δα, Δα'. But nonlinearity can be reduced and compensated for enough to make it meaningless in practice.

From the above equation it can be seen that the signs of the Angle .DELTA..alpha. And .DELTA..alpha. 'Are insignificant, since they relate to these signs behaves symmetrically. Only the magnitudes of the angles Δα and Δα 'are of Importance. An example of how the angles are at least approximate Compensation will be calculated below for a sensor configuration given where the fast axes of the phase delay elements 13,14 are aligned parallel to each other, so χ = 0 °.

FIG. 4 indicates for χ = 0 °, Δα = Δα '= 0 ° and different angles ε = ε' the relation between the direct measurement signal Δ _S and twice the Faraday phase delay angle, 2Δφ _F , calculated according to the above equation , Δ _S is normalized to 2φ _F.

It can be clearly seen that the nonlinearity increases for larger angles ε. The size of the non-linearities is in the per thousand to percent range. For ε = 13 ° the relative nonlinearity is -0.21% at 2φ _F = 40 ° and -0.92% at 2φ _F = 90 °. These nonlinearities are calculated according to {Δ S / (2φ F ) [2φ F = 40 °] - Δ S / (2φ F ) [2φ F = 0 °]} / Δ S / (2φ F ) [2φ F = 0 °] respectively. {Δ S / (2φ F ) [2φ F = 90 °] - Δ S / (2φ F ) [2φ F = 0 °]} / Δ S / (2φ F ) [2φ F = 0 °], ie from the difference of Δ _S / (2φ _F ) at 2φ _F = 40 ° or 2φ _F = 90 ° and Δ _S / (2φ _F ) at 2φ _F = 0 °, normalized with Δ _S / (2φ _F ) at 2φ _F = 0 °.

FIG. 5 shows, for the case ε = ε '= 0 ° for different angles Δα = Δα', the relationship between the direct measurement signal Δ _S and twice the Faraday phase delay angle, 2Δ, calculated in accordance with the above equation for χ = 0 ° _S , on. Δ _S is normalized to 2φ _F. It can be clearly seen that the nonlinearity increases for larger angles Δα. The size of the nonlinearities is in the per thousand range. It is noticeable that the curvature of the curves in FIG. 5 is opposite to the curvature of the curves in FIG. This opens up the possibility of said inventive compensation of nonlinearities attributable to ε ≠ 0 ° and / or ε '≠ 0 ° by clever choice of the angles Δα, Δα'.

FIG. 6 shows, for the case ε = ε '= 13 ° and χ = 0 ° for different angles Δα = Δα', the relationship between the direct measurement signal Δ _S and twice the Faraday phase delay angle, 2Δ, calculated according to the above equation _S , on. Δ _S is normalized to 2φ _F. It can be clearly seen that for angles Δα of 5.85 °, the nonlinearity attributable to ε ≠ 0 ° is at least approximately compensated. In this way, the inventive angle Δα can be determined graphically.

However, a calculation of Δα as a function of ε is to be preferred. at a given phase delay angle ρ = ρ ', that is a given Angle ε = ε ', the inventive angle Δα = Δα' can be as follows Help calculate the equation given above:

In practice, the function Δ _S (ε = ε ', Δα = Δα', φ _F ) / 2φ _{F should be} independent of 2φ _F in particular for values of 2φ _F between 0 ° and 90 °. The angle Δα = Δα 'to be chosen for this results from Δ S (ε, Δα = Δα ', φ F = 0 °) / (2φ F ) = Δ S (ε, Δα = Δα ', φ F = 90 °) / (2φ F )

This equation can be solved numerically. Alternatively, for χ = 0 ° and small angles ε, the following analytical expression is also obtained: sin 2 (2Δα) ≈ sin 2 ε - 2 [(π-1) / (π-2)] sin 4 ε + ...

Higher order terms are omitted here. This results in an approximate solution for Δα = Δα ' Δα ≈ ± (1/2) arcsin {sin 2 ε - 2 [(π-1) / (π-2)] sin 4 ε} 1.2

For ε = 13 °, this equation yields Δα = ± 5.85 ° (see also FIG. 6). In this case, the ratio Δ _S / 2φ _F between 2φ _F = 0 ° and 2φ _F = 90 ° varies by less than 0.02%. It therefore varies by almost a factor of 50 and thus is more than one and a half orders of magnitude less than the above-mentioned 0.92% in the case of Δα = Δα '= 0 °. If an even more extensive linearization is desired, then this variation can be minimized by means of an iterative method in a predetermined value range of the variable 2φ _F , for example between 2φ _F = 0 ° and 2φ _F = 90 °. For ε = 13 °, one then obtains Δα = ± 5.9 ° as the optimum angle. The procedure in the case of a sensor with χ ≠ 0 ° is completely analog. Of particular interest is the case of orthogonal aligned fast axes of the phase delay elements 13,14, ie χ = 90 °. For χ = 90 °, with otherwise unchanged parameters, an optimal compensation of the nonlinearities Δα = Δα '= ± 6.8 ° results.

In addition to the discussed in connection with Figure 2 embodiments Many other variants are possible. The light pipe elements 11, 12 can also be used as other types of polarization preserving optical Be formed fibers such as so-called panda fibers, Bowtie fibers or fibers with an additional, inner, elliptical Cladding (fiber coat). Alternatively, it is also conceivable that the first linearly polarized Light waves 3,3 'directly or by means of a lens or an optical Module in the phase delay elements 13,14 initiate. Then you would be the light pipe elements 11,12 air or vacuum, or the lens or the optical assembly. As main axes of the light guide elements 11,12 always the axes are referred to, by the polarization vectors of the first linearly polarized light waves 3,3 'are given.

The optical connections between the phase delay elements 13,14 and the light pipe elements 11,12 or the sensor element 15 can be direct connections, as for example created by welding together using a so-called Splice device become. Or they are connections via an intermediate medium, for example a gel, glue or piece of fiber or an optical assembly. Or the coupling of light waves takes place through a vacuum or through a gas instead.

The phase delay elements 13, 14 may be optical fiber pieces with geometrically induced birefringence, for example by an elliptical one Core, or with stress-induced birefringence, such as Bowtie or panda fibers or fibers with an inner elliptical Coat. They can also be used as loops of ordinary single-mode fibers be formed with a round core. Here is the phase delay generated via the birefringence, which is caused by the fiber curvature becomes. Furthermore, λ / 4 plates are also conceivable. The phase delay angle ρ, ρ 'can be subtended by angles ε, ε' from any odd number Multiples of 90 °. The angles ε, ε 'are preferably given that they are just so big that the temperature dependence the Verdet constants of the sensor element 15 by the temperature dependence the phase delay elements 13,14 compensated becomes. This can result in positive as well as negative angles ε, ε '.

The angles ε and ε 'can be of different sizes. There is still one for you given angle χ in general many different pairs of angles Δα, Δα ', which leads to an at least approximate compensation of ε ≠ 0 ° and / or ε '≠ 0 ° resulting nonlinearities. Nevertheless, it is the choice of Δα then still depends on ε, but Δα then depends in addition from ε 'and Δα' and from χ. One can also say that Δα and Δα 'in Depending on at least the angles ε and ε 'are selected. Of the Angle χ is still added as an influence.

The sensor element 15 may, as indicated above coil-shaped, preferably in several turns, the current conductor S include. However, fractions of a turn are also possible, and differently curved or non-curved sensor elements 15 can also be used. Preferably, the sensor element 15 consists of an optical fiber which is free of mechanical stresses, as described in EP 0 856 737 A1. Particularly advantageous is the use of such a stress-free sensor fiber 15 together with a temperature-dependence-compensating phase delay element 13, 14, as described in EP 1 115 000. Such a current or magnetic field sensor according to the invention has virtually no temperature dependence, but has a linear relationship between a current I to be measured and the instantaneous measurement signal ΔΦ _S.

In addition to magneto-optically active fibers and solid glasses or magneto-optical crystals, such as yttrium-iron garnet, Y ₃ Fe ₅ O ₁₂ , can be used as a sensor element 15. Especially when the current or magnetic field sensor is used for the local measurement of magnetic fields, these variants are advantageous. The sensor element 15 must be operatively connected to the magnetic field to be measured, preferably at a location where the magnetic field is large, so that the elliptically polarized light waves 6, 6 'experience the greatest possible magneto-optically induced phase shift due to the magnetic field. Furthermore, it is also possible to use a plurality of sensor elements 15 in a sensor head 1.

For the angles ε, ε 'and Δα, Δα' the conditions 0 ° <ε, ε '<90 ° and 0 ° <Δα, Δα '<45 °, where appropriate also Δα' = 0 ° and / or ε '= 0 be can. Since, as stated above, the sign of Δα does not matter, one can limit oneself to positive Δα. Be ε, ε 'for the above Temperature compensation selected, resulting from the Fiber materials available today have values of up to about 20 ° for ε, ε ' ε = ε 'and χ = 0 ° or χ = 90 °. Thus, preferably angles ε are smaller than about 30 ° before. For such ε, angles Δα of up to about 10 ° result. As mentioned above, with Δα ≠ Δα ', it is possible to have different pairs Δα, Δα 'to choose according to the invention, so that at ε ≈ 30 ° and inventive Angle Δα greater than 10 ° are possible.

As transmitter-evaluation unit 2 are interferometric as well as polarimetric Detecting variants possible. From the prior art are various Possibilities for evaluating the immediate measurement signals known. In the example of FIG. 2, each one was elliptically polarized Light waves 6 as a reference signal for the other elliptically polarized Light waves 6 'used, both of which are influenced by the electric Electricity I or the magnetic field were exposed. But it is also possible to measure the magneto-optically induced phase shift without each other to use different elliptically polarized light waves 6 and 6 '. For example, within the transmitter-evaluation unit 2 linear polarized light waves are generated, which induced no magneto-optically Phase shift, and opposite to which the magneto-optically induced phase shifts of the elliptically polarized light waves 6 or 6 'determine.

As the light source 20 is typically a low-coherence semiconductor source used, such as a superluminescent diode, a multimode laser diode, a laser diode operated below the laser threshold, or a luminescent diode (LED), preferably with wavelengths around 800 nm, 1300 nm or 1550 nm. But different wavelengths, for Example from the ultraviolet, the visible or the infrared range are usable.

In the manufacture of a sensor head so are the Angle Δα, Δα 'as a function of the angles ε, ε' in such a way chosen that the nonlinearities mentioned significantly reduced or even be compensated at least approximately. This can be, for example done in one of the ways described above. You can therefore also speak of a defined angle Δα, selected according to the invention becomes. This clearly borders him on chance, for example Tolerance-related angles Δα, preferably as small as possible, ie are about 0 °. In carrying out the invention, it is irrelevant whether for example due to manufacturing tolerances, one of the optimum angle Δα slightly different angle is realized. It is essential that one defined angle .DELTA..alpha. is chosen with the proviso that said Reduce nonlinearities, and that a corresponding result is achieved.

LIST OF REFERENCE NUMBERS

1

sensor head

11

first light pipe element

12

second light pipe element

13

first phase delay element

131

first end of the first phase delay element

132

second end of the first phase delay element

14

second phase delay element

141

first end of the second phase delay element

142

second end of the second phase delay element

15

sensor element

2

Transmission evaluation unit

20

light source

21

fiber coupler

22

fiber polarizer

24

second fiber coupler

25

phase modulator

26

detector

27

signal processor

28

Measured value output

3, 3 '

first linearly polarized light waves

4.4 _x , 4 _y , 4 ', 4 _x ', 4 _y '

second linearly polarized light waves

5, 5 '

third linearly polarized light waves

5a, 5a '

fourth linearly polarized light waves

6, 6 '

elliptically polarized light waves

I

electric current, amperage

N

Number of turns

S

conductor

V

Verdet constant

Δα

Angle by which the coupling angle into the (first) phase delay element deviates from 45 °

Δα '

Angle by which the coupling angle into the (second) phase delay element deviates from 45 °

Δ _S

differential phase shift in Sagnac configuration, immediate measurement signal with Sagnac configuration

ε

Angle by which the phase delay angle ρ of the (first) Phase delay element of an odd number Multiple deviates from 90 °

ε '

Angle by which the phase delay angle ρ 'of the (second) Phase delay element of an odd number Multiple deviates from 90 °

φ _F

Faraday phase shift, φ _F = VNI

ρ

Phase delay angle of the (first) phase delay element

ρ '

Phase delay angle of the (second) phase delay element

χ

angle

Claims (11)

1. Method for production of an optical current or magnetic field sensor, which comprises a transmission evaluation unit (2) and a sensor head (1), in which case the transmission evaluation unit (2) can produce light at a wavelength λ, and in which case the sensor head (1) comprises a first optical fibre element (11), a second optical fibre element (12), a first phase delay element (13), a second phase delay element (14) and a sensor element (15), in which case each of the two optical fibre elements (11, 12) each has at least one major axis (x', y'), in which case each of the two phase delay elements (13, 14) each has at least one major axis (x, y), and in which case elliptically polarized light waves (6; 6') can propagate in the sensor element (15) and are subjected to a magnetooptically induced phase shift as a result of an electric current or magnetic field to be measured,
   with a first end (131) of the first phase delay element (13) being optically connected to the first optical fibre element (11), and a second end (132) of the first phase delay element (13) being optically connected to a first end of the sensor element (15),
   with a first end (141) of the second phase delay element (14) being optically connected to the second optical fibre element (12), and a second end (142) of the second phase delay element (14) being optically connected to a second end of the sensor element (15),
   with the transmission evaluation unit (2) being optically connected to the first optical fibre element (11) and to the second optical fibre element (12),
   with the first phase delay element (13) being designed such that its phase delay angle p differs from an odd-numbered multiple of 90° by an angle ε where ε ≠ 0° and -90° < ε < 90°, and
   with the second phase delay element (14) being designed such that its phase delay angle ρ' differs from an odd-numbered multiple of 90° by an angle ε' where ε' ≠ 0° and -90° < ε' < 90°, and
   with the first optical fibre element (11) being arranged relative to the first phase delay element (13) such that the at least one major axis (x', y') of the first optical fibre element (11) includes an angle which differs from 45° by an angle Δα with the at least one major axis (x, y) of the first phase delay element (13),
   characterized in that the angle Δα is defined by: 0° < Δα < 45° and in that the angle Δα is chosen as a function of at least the angles ε and ε' such that any non-linearities in the relationship between the magnetooptically induced phase shift of the elliptically polarized light waves (6; 6') and the electric current or magnetic field to be measured, which occur when the current or magnetic field is measured by means of the optical current or magnetic field sensor, are less than in the situation Δα = 0°.
2. Production method according to claim 1, characterized in that the angle Δα is chosen as a function of at least the angle ε such that the non-linearities in the relationship between the magnetooptically induced phase shift of the elliptically polarized light waves (6; 6') and the electric current or magnetic field to be measured, which occur when the current or magnetic field is measured by means of the optical current or magnetic field sensor, are reduced by a factor of at least 3 in comparison to the situation Δα = 0°.
3. Production method according to Claim 1,
   characterized in that phase delay elements (13, 14) are used which together have a temperature dependency which at least approximately compensates for any temperature dependency of a Verdet constant of the sensor element (15).
4. Production method according to Claim 3,
   characterized in that the two phase delay elements (13, 14) are designed such that their phase delay angles p, p' are the same, and in that the second optical fibre element (12) is arranged relative to the second phase delay element (14) such that the at least one major axis (x', y') of the second optical fibre element (12) includes an angle which differs from 45° by an angle Δα', where 0° < Δα' < 45°, with the at least one major axis (x, y) of the second phase delay element (14), and
   in that the angles Δα and Δα' are chosen to be the same.
5. Production method according to Claim 1,
   characterized in that polarization-maintaining fibres are used as the optical fibre elements (11, 12), and in that these are connected to the phase delay elements (13, 14).
6. Production method according to Claim 1,
   characterized in that a piece of fibre with an elliptical core is used as at least one of the two phase delay elements (13, 14) and is designed such that its phase delay angle ρ, ρ' differs from 90° by the angle ε, ε'.
7. Production method according to Claim 1,
   characterized in that a sensor element (15) is used which can be arranged such that it has an electrical conductor (S) in the form of a coil.
8. Production method according to Claim 7,
   characterized in that a magnetooptically active fibre which has a round core cross section and is virtually free of mechanical stresses is used as the sensor element (15).
9. Production method according to Claim 1,
   characterized in that the magnitude of the angle ε is chosen to be less than 30°, and in that the angle Δα is chosen to be less than 10°.
10. Optical current or magnetic field sensor, comprising a transmission evaluation unit (2) and a sensor head (1), in which case the transmission evaluation unit (2) can produce light at a wavelength λ, and in which case the sensor head (1) comprises a first optical fibre element (11), a second optical fibre element (12), a first phase delay element (13), a second phase delay element (14) and a sensor element (15), in which case each of the two optical fibre elements (11, 12) each has at least one major axis (x', y'), in which case each of the two phase delay elements (13, 14) each has at least one major axis (x, y), and in which case elliptically polarized light waves (6; 6') can propagate in the sensor element (15) and are subjected to a magnetooptically induced phase shift as a result of an electric current or magnetic field to be measured, with a first end (131) of the first phase delay element (13) being optically connected to the first optical fibre element (11), and a second end (132) of the first phase delay element (13) being optically connected to a first end of the sensor element (15), with a first end (141) of the second phase delay element (14) being optically connected to the second optical fibre element (12), and a second end (142) of the second phase delay element (14) being optically connected to a second end of the sensor element (15), with the transmission evaluation unit (2) being optically connected to the first optical fibre element (11) and to the second optical fibre element (12),
    with the first phase delay element (13) being designed such that its phase delay angle ρ differs from an odd-numbered multiple of 90° by an angle ε where ε ≠ 0° and -90° < ε < 90°, and with the second phase delay element (14) being designed such that its phase delay angle p' differs from an odd-numbered multiple of 90° by an angle ε' where ε' ≠ 0° and -90° < ε' < 90°, and
    with the first optical fibre element (11) being arranged relative to the first phase delay element (13) such that the at least one major axis (x', y') of the first optical fibre element (11) includes an angle which differs from 45° by an angle Δα with the at least one major axis (x, y) of the first phase delay element (13),
    characterized in that the angle Δα is defined by: 0° < Δα < 45° and in that the angle Δα is chosen as a function of at least the angles ε, ε' such that any non-linearities in the relationship between the magnetooptically induced phase shift of the elliptically polarized light waves (6; 6') and the electric current or magnetic field to be measured, which occur when the current or magnetic field is measured by means of the optical current or magnetic field sensor, are reduced by a factor of at least 3 in comparison to the situation Δα = 0°.
11. Method for measurement of an electric current or a magnetic field, with first linearly polarized light waves (3) being produced in a transmission evaluation unit (2) and being injected into a first end (131) of a first phase delay element (13),
    with the first linearly polarized light waves (3) exciting second linearly polarized light waves (4; 4_x, 4_y) in the first phase delay element (13), which pass through the first phase delay element (13) and in consequence are subject to a phase shift through an angle ρ with respect to one another,
    with the second linearly polarized light waves (4; 4_x, 4_y) stimulating elliptically polarized light waves (6) in a sensor element (15) whose first end is connected to a second end (132) of the first phase delay element (13), which elliptically polarized light waves (6) are subjected to a magnetooptically induced phase shift produced by the electric current (I) or the magnetic field,
    with the elliptically polarized light waves (6) then being injected into a second phase delay element (14), whose second end (142) is connected to a second end of the sensor element (15),
    with the elliptically polarized light waves (6) exciting third linearly polarized light waves (5; 5_x, 5_y) in the second phase delay element (14), which pass through the second phase delay element (14) and in consequence are subjected to a phase shift through an angle ρ' with respect to one another,
    with the third linearly polarized light waves (5; 5_x, 5_y) being emitted from a first end (141) of the second phase delay element (14) and exciting fourth linearly polarized light waves (5a) which are supplied to the transmission evaluation unit (2),
    with the fourth linearly polarized light waves (5a) being detected, and the measured data being evaluated, in the transmission evaluation unit (2),
    with the angle ρ differing from an odd-numbered multiple of 90° by an angle ε, where ε ≠ 0° and -90° < ε < 90°, and with the angle ρ' differing from an odd-numbered multiple of 90° by an angle ε', where ε' ≠ 0° and -90 < ε' < 90° and
    with a polarization axis (x', y') of the first linearly polarized light waves (3) being arranged relative to a polarized axis (x, y) of the second linearly polarized light waves (4), and/or a polarization axis (x, y) of the third linearly polarized light waves (5) being arranged relative to a polarization axis (x', y') of the fourth linearly polarized light waves (5a) such that the corresponding polarization axes include an angle which differs from 45° by an angle Δα,
    characterized in that the angle Δα is defined by: 0° < Δα < 45° and in that the angle Δα is chosen as a function of at least the angles ε, ε' such that any non-linearities in the relationship between the magnetooptically induced phase shift of the elliptically polarized light waves (6; 6') and the electric current (I) or magnetic field to be measured, are less than in the situation Δα = 0°.

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